1. Field of the Invention
The present invention relates generally to a variable transmittance birefringent device, and specifically to a variable reflectance vehicle mirror which electrically controls reflectivity thereof by use of a birefringent medium.
2. Description of Related Art
A familiar hazard for the driver of a vehicle that is being xe2x80x98blindedxe2x80x99 by the glare of light beams from the headlights of a following vehicle, such beams being reflected by the rearview mirror of the driver""s vehicle. In order to avoid this glare, prismatic rearview mirrors are used in the interior of a vehicle which can be switched from a high to a low reflecting state by use of a manual lever located on the mirror. Under ordinary driving conditions, the high reflecting state of the mirror is used to provide optimal rear visibility. At night, the interior mirror is often switched to its low reflecting state to prevent the driver from being blinded by the headlights of following vehicles. The low reflectivity state of the mirror typically exhibits non-spectral selectivity, where the background of an image viewed in the low reflectivity state of the prismatic mirror would be color neutral.
With the advent of electro-optic technology, it has become possible to achieve continuous variability in reflectivity in rearview mirrors for motor vehicles. This variability has been achieved, for example, through the use of electrochromic devices, wherein the intensity of light is modulated by passing the light through an electrochromic (EC) cell. Electrochromism describes materials which change color when their composition is changed by use of an electrochemical cell, which enables the materials to be reversibly colored or bleached. In such devices, the EC cell includes an electrochromic medium disposed between two conductive electrodes which undergoes Electrochromism when potential differences are applied across the two electrodes. Antiglare mirrors for use in vehicles using a solution-phase or gel-type EC cell and thin-film type EC cell have been conventionally proposed. The solution-phase or gel-type EC cell, which is formed of a liquid or gel material such as biologen compound sealed between glass substrates, uses coloring due to oxidation/reduction of the biologen compound when a voltage is applied to the glass substrates. The thin-film type EC cell, which is formed of a transient metal oxide such as WO3 vapor-deposited on the glass substrates, uses coloring due to injection of hydrogen and metal ions into and emission thereof when a voltage is applied to the glass substrates. Therefore, whether the anti-glare mirror uses the solution phase type EC, the gel-type EC or the thin-film type EC cell, as the applied voltage becomes high, its mirror reflectivity decreases due to the coloring of the EC cell, and when the applied voltage is removed, the reflectivity is restored to its initial value.
Mirrors employing EC cells have several drawbacks. In order to provide a rapid change in intensity and uniformity in the coloring of the EC cell, very large electrical contacts extending substantially along the entire length of the top and bottom surfaces of the mirror are required. Thus, these mirrors require bevels or rims to cover and insulate these electrical contacts, which increase the size, cost, and complexity of the mirrors. Furthermore, use of a gel-type or solution-phase EC cell is undesirable due to the likelihood of the electrochromic medium leaking out of mirror should the mirror become damaged. The leakage of the electrochromic medium can not only be dangerous to passengers in the vehicle who are exposed to the electrochromic medium, but leakage of the electrochromic medium can further render the mirror non-functional. The constituency of the EC cell is damaging to the interior dashboard and exterior finish of the vehicles on which they are mounted should the mirror become cracked or the cell ruptured by impact or collision causing the contents to leak out. Electrochromic devices may also exhibit deleterious performance when exposed to ultraviolet radiation over prolonged periods of time, which may be linked to a variety of sources including a potential propensity for photochromism to occur.
Vehicles are typically equipped with an interior rearview mirror as well as exterior mirrors outside of the driver and passenger doors. In order to prevent the driver from being subjected to a glare from rearward light reflected from any of these mirrors, each of the mirrors must have a controlled reflectivity. In current systems having mirrors possessing EC cells, it is known to utilize a singular drive circuit to control all of the mirrors. This drive circuit is typically housed with the interior mirror, requiring a dedicated wire harness and specific input voltage to control and activate the EC cells on the exterior mirrors of the vehicle. All of the EC cells within the system are uniformly activated by the drive circuit, even when the dazzle or glare is only coming from one of the mirrors. This uniform activation of all of the mirrors unnecessarily reduces the image quality of the other mirrors where no dazzle or vision impairing is present.
Mirrors are also known which make use of the properties of nematic liquid crystals which are normally transparent to light but which when subjected to a sufficient electrical voltage beyond a certain threshold, present a state of turbulence so that the light is attenuated to an increasing degree as the applied electric field is increased. Upon suppressing the applied electric field, the liquid crystal returns to the transparent state. Using such mirrors, therefore, it is possible to obtain selectively a high or a low reflecting power, according to whether the electrical voltage acting on the liquid crystal is lower or greater than said threshold. The chief drawback of conventional liquid crystal mirrors is that when the mirrors work in conditions of weak reflecting power the images of objects viewed by reflection in the mirror are considerably dimmed. In these types of liquid crystal mirrors, either Chromium or Rhodium plated or deposited surfaces are used as the reflective medium. These mediums offer only a 50 to 55% reflectivity in the visible wavelengths, thus reducing the image brightness even in the non-activated state.
Some examples of these prior art liquid crystal mirrors are described in U.S. Pat. No. 3,862,798 issued to Hopkins and U.S. Pat. No. 4,200,361 issued to Malvano. These patents describe nematic liquid crystal devices having a liquid crystal film sandwiched between front and rear transparent electrode surfaces, where the nematic liquid crystal mirrors of these types are typically only capable of transmitting 50% of available light. With the standard 3% loss of transmission between the air to substrate interface and the fact that the light, in reflection, must travel back through the devise, there is an additional 6% loss of transmission, not with standing the liquid crystal film absorption. This rendered mirrors employing typical liquid crystal cells capable of, at best, 44% transmission of the incident light. This phenomenon directly resulted from the use of uncompensated-for polarized light in the reflective devise. Aside from typically exhibiting poor reflectivity, mirrors employing conventional liquid crystal materials have also generally suffered from having an insufficient viewing angle, low contrast ratios, and variable response times with temperature fluctuation.
The foregoing shortcomings and disadvantages of the prior art are alleviated by the present invention that provides a variable reflectance mirror employing a super-twisted nematic (STN) liquid crystal cell to control reflectivity. The STN liquid crystal cell includes a pair of transparent electrically insulating plates serving as front and rear plates for the liquid crystal cell, where these plates may comprise glass or an optical grade, stable synthetic plastic. The facing surfaces of each of these plates are coated with a transparent metal oxide film, which acts as an electrode. A layer of STN liquid crystal material is formed in the area between the electrodes, where a polymer alignment layer is formed over the conductive layers and in contact with the STN liquid crystal material. The alignment layer is treated in a way so as to orient the STN liquid crystal material to possess a twist angle between approximately 180xc2x0 and approximately 270xc2x0. A pair of crossed polarizers are respectively positioned on the outer surfaces of the front and rear plates. A layer of reflective material is further formed adjacent to the outer surface of the polarizer adjacent to the rear plate, where the reflective layer is affixed to the rear polarizer by a bonding layer. The variable reflectance mirror includes a front transparent cover element which is affixed to the front polarizer by a bonding layer and a rear transparent cover element positioned adjacent to the reflective layer, wherein the front and rear transparent cover elements form the outer surfaces of the mirror.
The conductive layers are connected to a voltage source to apply an electrical bias to the STN liquid crystal layer, where the transmitivity of the STN liquid crystal layer to light can be varied by varying the electrical bias applied across the conductive layers. When little or no voltage is applied across the conductive layers, the liquid crystal layer is essentially transparent to light. As a voltage is applied across the transparent electrodes, the twist angle of the STN liquid crystal layer changes to make the layer more opaque and to scatter light. The degree of opaqueness achieved in the STN liquid crystal layer is proportional to the amount of voltage applied across the transparent electrodes. The degree of reflection provided by the variable reflectance mirror is adjusted by adjusting the electrical bias applied across the STN liquid crystal layer.
A control circuit is connected to the STN liquid crystal cell to control the electrical bias applied across the STN liquid crystal layer. A rear light detecting sensor and an ambient light detecting sensor are further connected to the control circuit for determining the intensity of the light impinging on the variable reflectance mirror from the rear of the vehicle. The control circuit adjusts the reflectivity of the variable reflectance mirror based upon the intensity of the light measurements made by the rear light detecting sensor and an ambient light detecting sensor.